Your Speedo=Tach problem is a weird one. Never heard of it before. As far as I know, none of the speedometer wiring is under the hood, where the bad insulation is found. If you have crumbling insulation, it's usually pretty obvious. I parted out an 86 and some of the wires were completely bare. Look at the wires to the injectors, oil pressure switch, around the gray block connector on the RH side of the firewall, and the ones to the alternator and the headlights.

It might be worth getting a used OEM coil from a junkyard (shouldn't be very expensive) and trying that.

The bulb warning problem is probably a bad ground, unrelated to the Speedo=Tach. Since it seems to be associated with the brake lights, clean the ground connections to the taillights and see if it helps. Aleekat is correct that having two different bulbs can cause this to happen too. Make sure both bulbs are the same brand and part number.